Buildbot represents the status of a step, build, or buildset using a set of
numeric constants. From Python, these constants are available in the module
buildbot.status.results, but the values also appear in the database and in
external tools, so the values are fixed.

This function takes two status values, and returns the "worst" status of
the two. This is used (with exceptions) to aggregate step statuses into
build statuses, and build statuses into buildset statuses.